1. Field of the Invention
The present invention relates to a dielectric resonant apparatus, and more particularly to a dielectric resonant apparatus for use in the microwave or millimeter wave range.
2. Description of the Related Art
A dielectric resonator having low phase noise and high stability of resonant frequency is used as a resonator or in an oscillator in a high-frequency range such as a microwave or millimeter wave range.
In Laid-open Japanese Patent Application No. 8-265015, the assignee of the present application has presented a module in which electrodes are arranged on both main surfaces of a dielectric sheet to form a dielectric resonator on a part of the sheet. The electrodes arranged on the dielectric sheet serve as ground potentials; and a microstrip arranged on another dielectric sheet is stacked on the dielectric sheet. This arrangement is used in a high-frequency module such as a VCO.
In addition, a similar type of high-frequency module has been presented in Japanese Patent Application No. 8-294087 and the co-pending U.S. patent application Ser. No. 08/965,464. FIGS. 19 and 20 illustrate the structure of the high-frequency module. Is should be noted that this high-frequency module was not laid-open to the public at the time of filing of the Japanese Application No. 10-42017 on which the present application is based. Thus, the inventors do not deem the high-frequency module of FIGS. 19-20 to be prior art with respect to the present invention.
In FIG. 19, reference numeral 1 denotes a dielectric sheet. An electrode is formed on each of two main surfaces of the dielectric sheet 1. Each electrode has an opening formed at a location corresponding to the location of the opening of the other electrode (reference numeral 4 denotes one opening). The part defined by the electrode openings serves as a dielectric resonator. A circuit board 6, on a surface of which a circuit including microstrip lines is formed, is placed on the upper surface of the dielectric sheet 1. On the circuit board 6, there are also provided coupling lines 11 and 12 at locations which allow the coupling lines 11 and 12 to be coupled with the dielectric resonator formed in the electrode openings 4.
In the example shown in FIG. 20, electrodes each having an opening formed at locations corresponding to each other (reference numeral 5 denotes an opening formed in one electrode) are disposed on two respective main surfaces of a dielectric sheet 1 such that the part defined by the electrode openings serves as a dielectric resonator. The dielectric sheet 1 is placed on a circuit board 6 such that the dielectric resonator is coupled with a transmission line 11 or 12 formed on the circuit board 6. A spacer is disposed between the dielectric sheet 1 and the circuit board 6 so that electrodes on the lower surface, in FIG. 20, of the dielectric sheet 1 are insulated from the electrodes 11 and 12 on the upper surface of the circuit board 6.
In dielectric resonators of the types described above in which electrodes each having an opening formed at locations corresponding to each other are disposed on respective two main surfaces of a dielectric sheet, almost all electromagnetic field is confined in the part defined by the electrode openings and thus electromagnetic energy is concentrated in that part. Therefore, strong coupling can be achieved by placing the coupling line at a proper location. Thus, the dielectric resonator can be used, for example, to realize an oscillator having a large oscillation frequency modulation width and/or large output power.
In the oscillators shown in FIGS. 19 and 20, the frequency modulation width varies depending on the external Q (Qe2) of the resonant circuit (coupling line 12) as shown in FIG. 16. As can be seen from FIG. 16, it is possible to greatly increase the frequency modulation width by reducing the external Q (Qe2).
FIG. 17 illustrates the relationship between the reflection coefficient of the resonant circuit and the external Q (Qe1) of the dielectric resonator and the band-reflection coupling line 11. From FIG. 17, it can be seen that the reflection coefficient of the resonant circuit increases if the external Q (Qe1) is reduced. Because the output increases with the increase in the reflection coefficient of the resonant circuit, it is possible to increase the output by reducing the external Q (Qe1).
FIG. 2 illustrates an electromagnetic field distribution in a dielectric resonator of the type in which the resonator is formed on a dielectric sheet in the manner described in FIG. 19 or 20. In FIG. 2, reference numerals 2 and 3 denote electrodes formed on the respective main surfaces of the dielectric sheet 1. The part defined in the circular openings 4 and 5 of the respective electrodes 2 and 3 serves as a TE010-mode dielectric resonator. In the conventional resonant circuit for use in an oscillator, the coupling lines 11 and 12 are disposed at locations slightly apart from the surfaces of the electrode openings 4 and 5 (hereinafter referred to as electrode opening planes) forming the dielectric resonator part. If the distance between the coupling lines and the electrode opening plane is increased, the electromagnetic field applied to the coupling lines decreases rapidly as can be seen from FIG. 2. This means that the degree of coupling decreases rapidly with the increase in the distance between the coupling lines and the electrode opening plane.
FIG. 18 illustrates the oscillation output as a function of the distance between the coupling lines and the electrode opening plane (wherein the distance is measured in a direction perpendicular to the electrode opening plane). As can be seen from FIG. 18, if the distance between the coupling lines and the electrode opening plane is reduced, then the external Q decreases and the output increases.
However, in the dielectric resonant apparatus shown in FIG. 19 or 20, it is impossible to reduce the distance between the coupling lines and the electrode opening to a value smaller than a practical limit. That is, in the example shown in FIG. 19, in order to decrease the distance from the electrode opening plane of the electrode opening 4 to the coupling lines 11 and 12, it is required to decrease the thickness of the circuit board 6 because the coupling lines 11 and 12 are disposed on the upper surface of the circuit board 6. However, the reduction in the thickness of the circuit board 6 is limited to a practically-possible minimum value. In the example shown in FIG. 20, it is required to reduce the thickness of the spacer. However, the spacer also has its minimum possible thickness. Besides, the reduction in the thickness of the spacer results in another problem that it becomes impossible to obtain a desired characteristic because the reduction in the thickness of the spacer produces a great change in the characteristic impedance of the lines 11 and 12.
Still another problem is the positioning accuracy of the coupling lines relative to the resonator. In the millimeter range, a very small change in the location of the coupling lines relative to the location of the resonator results in a large change in the characteristic. Therefore, high positioning accuracy is required. However, in the conventional dielectric resonant apparatus, the resonator and the coupling lines are produced separately by different processes, and thus it is difficult to achieve a required high positional accuracy.
It is an object of the present invention to provide a dielectric resonant apparatus including a resonant circuit using a dielectric resonator with a reduced external Q so that the dielectric resonant apparatus may be used, for example, to realize an oscillator having a large frequency modulation width and large output.
It is another object of the present invention to provide a dielectric resonant apparatus having high positional accuracy between a resonator and a coupling line and thus having a small characteristic variation.